It is proposed to study the regulation of the qualitative synthesis of collagen in pure populations of chick vertebral chondrocytes which have been induced to "dedifferentiate" by the addition to the medium of embryo extract or 5-bromo-2'-deoxyuridine, since it has been shown that this dedifferentiation is accompanied by a switch in the type of collagen which is synthesized. The studies on the mechanisms of the switch will focus on biochemical changes which occur during the cell cycle since experiments by the applicant have shown that in order to elicit the switch, cells must be capable of replication and the effect appears to be localized to a specific portion of the cell cycle. For these studies synchronized populations of cells will be employed. It is also proposed to characterize the newly-synthesized collagens from various differentiating tissues in cell or organ culture. Labelled, purified alpha chains will be examined for type and content of lysine, hydroxylysine and hydroxylysine glycosides. It is expected that meaningful temporal patterns will emerge which may suggest that these embryonic collagens and their modified derivatives serve important "instructional" roles in development.